Partial wall-flow filter and diesel exhaust system and method

ABSTRACT

A exhaust system and method for venting exhaust from an engine, such as a diesel engine, through an exhaust line coupled to the engine includes a first particulate filter disposed in the exhaust line and “close-coupled” with the engine, and a second particulate filter spaced a distance (d) from the first filter. The first particulate filter is “close-coupled” so that it operates in a passive regeneration mode to a greater extent than the second particulate filter. The first particulate filter may be a partial wall-flow filter including some plugged and some open channels. Some of the plugged channels may be plugged adjacent to an inlet end and others may be plugged adjacent to an outlet end. A partial wall-flow filter is also described having some unplugged flow-through channels and some plugged channels wherein some plugged channels are located adjacent to the inlet end and some are located adjacent to the outlet end.

BACKGROUND OF THE INVENTION

The present invention relates generally to wall-flow filters used to filter exhaust gases, and exhaust systems and methods incorporating such filters.

Diesel exhaust systems may include, for example, a diesel particulate filters (DPFs) for removing particulates, such as soot from diesel exhaust. Where multiple DPFs are used to remove particulates, these DPFs are typically arranged in close proximity to each other and housed within a common enclosure, such as taught in US Pat. App. No. 2004/0161373. The most widely used DPFs are wall-flow filter. The conventional wall-flow filter consists of a ceramic honeycomb substrate having longitudinal, parallel cell channels formed by a plurality of intersecting porous walls. The ends of the cell channels are typically plugged with a ceramic plugging cement to form a checkered pattern of plugs at the end faces of the honeycomb substrate. The cell channels of the filter typically have some ends plugged at an inlet end face of the honeycomb substrate, referred to herein as “inlet channels.” Likewise, typically, the cell channels also have the remaining ends plugged to form a checkered pattern of plugs at an outlet end face of the honeycomb substrate, herein referred to as “outlet channels.” In use, exhaust gas containing entrained soot particles enters into the inlet channels, flows through the porous walls (i.e., the wall-flow) and into the outlet channels, and exits through the outlet channels, with the porous walls retaining a portion of the particles contained in the exhaust. Filtration efficiencies greater than 90% have been realized with conventional wall-flow filters.

Conventional wall-flow filters may be cleaned out to prevent the filter from becoming blocked and to maintain a suitable pressure drop across the filter below a prescribed limit. Increase in pressure drop across the filter generally results in an increase in backpressure against the engine which, if not controlled, may lead to power loss. One known method for cleaning out the filter is to remove the soot trapped in the filter by thermal regeneration (hereinafter “regeneration”). The regeneration may be either “passive” or “active” or a combination thereof. In “passive” regeneration, the inlet temperature of the exhaust entering the filter is sufficiently high to itself initiate combustion of the soot trapped in the wall-flow filter. In “active” regeneration, the temperature of the filter is relatively low and additional energy input is required to raise the temperature of the exhaust (and the filter) to a level that would cause combustion of the soot trapped in the filter. Typically, the additional energy input is provided by post injection of fuel into the exhaust in combination with a diesel oxidation catalyst located upstream of the filter.

Diesel exhaust systems based on “active” regeneration have become the industry standard because they desirably operate at lower exhaust temperatures and assure suitable soot removal under different engine duty cycles by implementing regeneration. On the other hand, “active” regeneration comes with a fuel economy penalty. Additionally, there is the possibility of large temperature spikes during “active” regeneration which may be detrimental to the filter. Accordingly, systems which fewer regeneration event during operation are desired.

SUMMARY OF THE INVENTION

In view of the inefficiencies of the exhaust systems of the prior art, diesel exhaust after treatment systems operating to a larger extent in a passive regeneration mode may offer a competitive advantage in terms of fuel economy. In one broad aspect, the invention is an exhaust system adapted for venting exhaust gas from an engine, such as a diesel engine, through an exhaust line coupled to the engine. The exhaust system comprises a first particulate filter disposed in the exhaust line and “close-coupled,” i.e., located in close proximity to the engine. and a second particulate filter positioned inline with, and spaced a distance (d) from, the first particulate filter. The spacing between the first and second particulate filters is preferably such that the difference between the inlet temperature of the first filter (T₁) and the inlet temperature of the second filter (T₂), namely T₁-T₂, is 20° C. or greater. The first filter is “close-coupled” to the engine so that it operates at a temperature preferably sufficient to promote a significant amount of “passive” regeneration whereas, the downstream second filter operates in a cooler environment and, therefore, relies more on active regeneration for soot removal. The relative extent if passive regeneration undergone in the second filter may be substantially less than in the first (close-coupled) filter. In one implementation, the first particulate filter may have a first percentage of plugged channels and a second percentage of unplugged flow-through channels. According to additional embodiments, the plugged channels may be plugged adjacent to both an inlet end and an outlet end. In a preferred implementation, the second particulate filter is positioned inline with, and spaced a distance (d) of greater than or equal to 12 inches (30.5 cm) from, the first particulate filter. Optionally, the spacing may be such that the first particulate filter may include a first inlet temperature (T₁) and the second particulate filter includes a second inlet temperature (T₂), wherein a ratio of inlet temperatures (T₁/T₂) is greater than or equal to 1.1, or even greater than or equal to 1.15. According to further embodiments of the invention, the first and second particulate filters may be mounted into separate housings. A diesel oxidation catalyst may be included in the system between the filters or the oxidation catalyst function may be included in the first filter.

In another broad aspect, the invention is directed to an exhaust system, such as a diesel exhaust system, comprising a first particulate filter which is “close-coupled” to an engine, the first particulate filter being a partial wall-flow filter having a first percentage of plugged channels and a second percentage of unplugged flow-through channels; and a second particulate filter positioned inline with and spaced a distance (d) of greater than 12 inches (30.5 cm) from the first particulate filter. The second filter may include only plugged channels, and the first filter may be located relative to the engine such that it exhibits passive regeneration to a substantially greater extent than the second filter. The second filter preferably may also be subjected to “active” regeneration. The first and second filters are preferably housed in separate, spaced housings.

In another broad aspect, the invention is directed to a method of operating an exhaust system, such as a diesel exhaust system, comprising the steps of passing an exhaust gas through a first particulate filter disposed in an exhaust line wherein the first particulate filter includes a first inlet temperature (T₁), the first particulate filter having a first percentage of plugged channels and a second percentage of unplugged flow-through channels wherein the exhaust gas is first filtered; and passing the first filtered exhaust gas through a second particulate filter positioned inline with and spaced from the first particulate filter wherein the second particulate filter includes a second inlet temperature (T₂), wherein the first exhaust gas undergoes a second filtering, and a ratio of inlet temperatures (T₁/T₂) is greater than or equal to 1.1, or even greater than or equal to 1.15.

In additional embodiments, the invention is a partial wall-flow filter, adapted for use in a diesel exhaust system, comprising plugged channels and unplugged flow-through channels wherein the plugged channels include some channels that are plugged adjacent to an inlet end and other channels that are plugged adjacent to an outlet end.

Other features and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, described below, illustrate typical embodiments of the invention and are not to be considered limiting of the scope of the invention, for the invention may admit to other equally effective embodiments. The figures are not necessarily to scale, and certain features and certain view of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.

FIG. 1A and 1B are schematic diagrams of diesel exhaust systems according to embodiments of the invention.

FIGS. 2A and 2B are perspective views of a partial wall-flow filter for use in the exhaust systems of FIGS. 1A, 1B.

FIGS. 3A-3D are examples of plugging patterns which may be employed for the partial wall-flow filter according to embodiments of the invention.

FIG. 4A shows a partial plugging pattern applied at both ends of the partial wall-flow filter of FIGS. 2A, 2B.

FIG. 4B and 4C illustrate an alternative partial plugging pattern applied at both ends of the partial wall-flow filter.

FIGS. 5 and 6 illustrate alternative partial plugging patterns applied at both end faces of the partial wall-flow filter according to embodiment of the invention.

FIGS. 7-10 illustrate performance plots for various system configurations including the partial wall-flow filter according to embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in detail with reference to a few preferred embodiments, as illustrated in the accompanying drawings. In describing the preferred embodiments, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without some or all of these specific details. In other instances, well-known features and/or process steps have not been described in detail so as not to unnecessarily obscure the invention. In addition, like or identical reference numerals are used to identify common or similar elements.

FIG. 1A depicts an exhaust system 100, such as a diesel exhaust system, for venting exhaust from an exhaust manifold 105 of a diesel engine 107. The exhaust system 100, as shown, includes an exhaust line 102 with inlet end 101 and outlet end 103. The inlet end 101 is coupled to the diesel engine 107 through an exhaust manifold 105. The inlet end 101 may include a connection device 104, which may take on any suitable form. For example, the connection device 104 may be a flange that can be coupled to a similar flange on a connection portion 109 of the exhaust manifold 105. Although the exhaust line 102 is shown as being generally straight, in practice it may take on other profiles and may include straight and curved sections and/or sections of differing diameter.

The exhaust system 100 includes a first particulate filter 106 disposed adjacent to the inlet end 101 of the exhaust line 102 so as to be in a “close-coupled” position with respect to the engine 107 and, of course, also the exhaust manifold 105. In this “close-coupled” position, the first particulate filter 106 may take advantage of the higher incident exhaust temperatures to effect a substantially greater extent of “passive” regeneration of the captured soot, as compared to the downstream second filter. The term “close-coupled” as used herein, means the filter is in a location in the exhaust stream in close proximity to the engine 107, and, in particular, in close proximity to the combustion chambers of the engine, as measured along the exhaust stream. For example, “close-coupled” would be considered in close proximity of the engine 107, measured along the exhaust line, such that the temperature for at least some portion of the operating cycle exceeds 250° C. Preferably, for at least 50% of the operation, the inlet temperature (T₁) of the first filter exceeds 200° C. In one example shown in FIG. 1A, a turbocharger 111 is positioned in the exhaust line 102 and the first particulate filter 106 is positioned upstream of the turbocharger 111 such that the hot gases directly impinge upon the first filter 106. In a more preferred embodiment, the first filter 106 is located directly downstream of the turbocharger 111 (See FIG. 1B). In the close-coupled position, the first filter 106 may experience temperature conditions of 250° C. or greater for a substantial amount, greater than 10%, or even greater than 20%, of the operating cycle. These conditions promote a substantial amount of “passive” regeneration. To avoid unwanted damage to the filter, the inlet temperature T₁ should preferably not exceed about 400° C.

The exhaust system 100 of the invention further includes a second particulate filter 108 positioned in the exhaust line 102, and spaced a distance (d) from the first particulate filter 106. In the examples shown in FIG. 1A, 1B, the second particulate filter 106 may be positioned downstream of a turbocharger 111. Additional particulate filters may be positioned in the exhaust line 102, downstream of the second particulate filter 106 to meet desired filtration and backpressure requirements. The second particulate filter 106 may be preceded by an upstream diesel oxidation catalyst (DOC) 114, which may incorporate any known active catalytic species for purifying exhaust, such as catalytic species for oxidizing carbon monoxide, hydrocarbons, and soluble organic fraction of particulates, as is known in the art. If included, the DOC 114 may be located between the first 106 and second 108 filters, or more preferably between the first filter 106 and the turbocharger 111. The exhaust system 100 may further include devices such as diffusion and expansion cones 110, 112 at the inlet and outlet ends of the particulate filters 106, 108 to aid in achieving desired exhaust flow distribution in the particulate filters, and/or size and weight reductions in the exhaust line 102.

During normal operation of the engine, such as a diesel engine, exhaust from the engine 107 and exhaust manifold 105 passes sequentially through the first particulate filter 106, turbocharger 111 (if present), oxidation catalyst 114 (if present), and second particulate filter 108, as indicated by arrow 116 in FIG. 1A. Particulates in the exhaust are trapped inside the first and second particulate filters 106, 108 as the exhaust passes through them. In particular, part of the soot is trapped in the first filter, while some of the remaining soot is trapped in the second filter. The engine operating conditions and location of the first filter 106 relative to the engine 107 may be set such that the inlet temperature T₁ of the exhaust at the first filter 106 is sufficient to itself initiate combustion of soot trapped in the first filter 106, i.e., promote “passive” regeneration. Contrarily, the second filter 108 is spaced a distance (d) from the first filter 106 such that its inlet temperature T₂ is low in comparison to the inlet temperature T₁ of the first filter 106. In particular, the distance (d) is spaced generally such that a ratio of inlet temperatures (T1/T2) is greater than or equal to 1.1, or even greater than or equal to 1.15. Preferably, the spacing of the filters is such that the temperature difference, T₁-T₂, is 20° C. or more, or even the difference is 25° C. or more.

According to embodiments of the invention, the first particulate filter 106 has a relatively low pressure drop in comparison to the second particulate filter 108. In one example, the first particulate filter 106 is small enough to fit into the available space near the exhaust manifold 105, between the exhaust manifold 105 and the turbocharger 111, or just downstream of the turbocharger. According to additional aspects of the invention, the physical space (volume) needed to house the first particulate filter 106 may be relatively smaller than the space (volume) to house the second filter 108, because the second particulate filter 108 provides the additional volume needed to meet filtration requirements. In a preferred implementation, the second particulate filter 108 may be a conventional wall-flow filter, for example. However, a conventional wall-flow filter would typically not be suitable for use as the first particulate filter 106 because of the size and pressure drop requirements for a filter in a “close-coupled” position. In particular, the it is desirable that the first filter exhibit low pressure drop. Because of the low pressure drop requirement, the first particulate filter 106 may have a lower filtration efficiency than the second particulate filter 108. As an example, the first particulate filter 106 may have an initial filtration efficiency of less than about 80%. However, in certain configurations described herein, it is possible to achieve higher filtration efficiency in the first filter, such as no less than 40%, or even no less than 50%, or even no less than 60% or 70% or more, while the second particulate filter 108 preferably has filtration efficiency greater than about 80%, or even 90% or greater. In certain embodiments, the first filter exhibits initial filtration efficiency of greater than 40% but less than 80%, and the second filter exhibits initial filtration efficiency of greater than 90%. The first particulate filter 106 may be any suitable filter exhibiting one or more of the aforementioned characteristics. For example, the first particulate filter 106 may be a ceramic foam-type filter. Alternatively, the first particulate filter 106 may be a partial wall-flow filter. The partial wall flow filter being named because of having a combination of plugged and unplugged flow-through channels. In the unplugged flow-through channels, flow is straight through the channel, i.e., not through the wall. Thus, the “partial” indicates that only a part of the flow is through the wall. Partial wall-flow filters according to the invention exhibiting high porosity, greater than 45% and which have a combination of plugged and unplugged channels have been discovered to be most effective. Partial wall-flow filters having total porosities of 50% and more exhibit excellent filtration efficiency and low pressure drop.

FIGS. 2A and 2B depict an exemplary partial wall-flow filter 200 for use as the first particulate filter (106 in FIG. 1) which is located in close-coupled relationship to the engine 107. The partial wall-flow filter 200 of the invention includes a porous honeycomb substrate 202 having, for example, a generally cylindrical shape. Optionally, the transverse cross-section of the honeycomb substrate 202 may be circular, elliptical, square or may have other shape. The honeycomb substrate 202 has opposite ends 204, 206 and interior porous walls 208 extending between the ends 204, 206. The interior porous walls 208 define generally parallel flow channels 210, which also extend between the ends 204, 206. The channels 210 may have a square cross-section or other type of cross-section, e.g., triangle, circle, octagon, rectangle, hexagon or combinations thereof. The honeycomb substrate 202 is preferably made of a porous ceramic material, such as cordierite, aluminum titanate, or silicon carbide. The interior porous walls 208 of the honeycomb substrate 202 may include thereon an active catalytic species useful for passive regeneration of particulates accumulated in the porous walls.

For diesel exhaust systems, the porous walls 208 may incorporate pores having mean diameters in the range of 1 to 60 μm, more typically in the range of 10 to 50 μm, and the honeycomb substrate 202 may have a cell density between approximately 10 and 400 cells/in² (1.5 and 62 cells/cm²), more typically between approximately 100 and 320 cells/in² (15.5 and 49.6 cells/cm²). The thickness of the porous walls 208 may range from approximately 0.002 in. to 0.060 in. (0.05 mm to 1.5 mm), more typically between approximately 0.010 in. and 0.030 in. (0.25 mm and 0.76 mm), and the total porosity of the walls may be greater than 45%, or even greater than 50%, or even greater than 55%, or even greater than 60%.

Plugs 212 may be inserted at, for example, an end face of some of the channels 210, while the remaining channels 210 remain open (unplugged). This differs from the conventional wall-flow filter where all the cell channels are end-plugged. The unplugged, flow-through channels 210 a, which are open at both ends 204, 206 and are unplugged along their length are preferably evenly distributed among the plugged channels 210 b, or vice versa. Plugs may be included only at one of the ends 204, 206, or at both of the ends 204, 206. Optionally, the plugs may be included spaced in from the ends. In a partial wall-flow filter with plugs on only one side, partial filtration occurs by passage of exhaust through some of the walls, while some flow is straight through the filter. When the plugs are positioned adjacent to the outlet end of the filter, a pressure differential between plugged and unplugged, flow-through channels results in transfer of exhaust from plugged channels to unplugged, flow-through channels, and soot may be accumulated in the plugged channels. When plugs are positioned adjacent to the inlet end of the filter, exhaust enters the unplugged, flow-through channels and a pressure differential between the unplugged, flow-through channels and adjacent plugged channels forces some exhaust through the wall to exit through the outlet side of the plugged channels. Soot accumulation in this case occurs on the walls of the unplugged, flow-through channels. In one example, plugs are positioned only adjacent to the outlet end face of the filter. Filters with combinations of only outlet end plugs and unplugged, flow-through channels where the porosity is greater than 45%, greater than 50%, greater than 60% have been found to be particularly effective as a first filter promoting high soot capture in the first filter and exhibiting low pressure drops. Optionally, plugs may be include only adjacent to the inlet face.

In another example, plugs are positioned adjacent to both ends of the first filter. Thus, in this embodiment, a partial wall-flow filter is provided, comprising plugged channels and unplugged, flow-through channels wherein the plugged channels include some channels that are plugged adjacent to an inlet end, and other channels that are plugged adjacent to an outlet end. Preferred embodiments include relatively more plugs formed adjacent the outlet end than the inlet end. Embodiments including this configuration and high porosity, greater than 45%, have relatively minimal pressure drop as a function of soot loading. For example, FIG. 8 illustrates that configurations with 25% of the plugs located adjacent to the inlet end and 25% of the plugs located adjacent to the outlet end and porosity greater than 60% exhibits a pressure drop of less than 0.5 kPa from 0 to 2 g/l soot loading. Partial flow filters with high porosity, greater than 60%, with about 50% rear plugs also exhibit low pressure drop change as a function of soot loading. That being said, in some instances, it may be desirable to include a larger percentage of plugs adjacent to the inlet end that the outlet end.

This partial flow embodiment is demonstrated, for example, in FIGS. 2A and 2B, where the unplugged, flow-through channels are designated 210 a, the plugged channels having plugs located at the end 204 are designated 210 c, and the plugged channels having plugs at the end 206 are designated 210 b.

In the partial wall-flow filter 200, soot accumulates on the porous walls 208 as exhaust passes through the filter. This accumulation of soot decreases the permeability of the walls 208 and reduces exhaust flow to channels adjacent to the unplugged, flow-through channels 210 a. Thus, the ability of the partial wall-flow DPF 200 to capture soot decreases as soot is accumulated in the filter. One advantage of a filter which decreases in filtration efficiency is that a maximum soot load can be established for the filter and overloading of soot in the filter is less likely to occur in a partial wall-flow filter. In conventional wall-flow filters, filtration efficiency generally increases as soot load accumulation on the porous walls increases, making the filter more susceptible to soot overload. Soot overload is undesirable because maximum temperatures encountered in the filter during regeneration are directly proportional to soot load. The partial wall-flow filter 200 has a built-in protection against high temperature excursions resulting from soot overload.

Various examples of partial plugging patterns will now be described. However, these examples should not be construed as limiting the invention as otherwise described herein.

FIG. 3A shows a partial plugging pattern 300 wherein the number of plugged channels 302 is greater than the number of unplugged, flow-through channels 304. Also, the unplugged, flow-through channels 304 may be evenly distributed among the plugged channels 302. As an example, the ratio of the number of plugged channels 302 to total number of channels, expressed as a percentage, may constitute greater than 50%, or even greater than 60%, or even 75%. Plugged channels for this configuration are preferably located at the outlet end. Having a higher percentage of plugs at the outlet end coupled with high porosity, greater than 45%, or even greater than 50% appears to provide initial filtration efficiency greater than 40%, or even 50%, or even 60% or more (See FIG. 7).

FIG. 3B shows an alternative plugging pattern 306 wherein the number of unplugged, flow-through channels 308 is greater than the number of plugged channels 310. Again, the plugged channels 310 may be evenly distributed among the unplugged, flow-through channels 308. As an example, the number of unplugged, flow-through channels 308 may constitute greater than 50% of the total number of channels, or even greater than 60%, or even 75% or more of the total number of channels.

FIG. 3C shows a partial plugging pattern 312 for the first filter including unplugged, flow-through channels 314 and plugged channels 316, wherein the hydraulic diameters of the plugged and unplugged, flow-through channels 314, 316 are different. In particular, the hydraulic diameter of the plugged channels 316 are larger than the hydraulic diameter of the unplugged, flow-through channels 314. Again, the plugged channels are preferably located adjacent to the outlet end. In particular, an area ratio of the plugged area to open area of the filter is preferably greater than 1.2.

FIG. 3D illustrates a partial plugging pattern 318 including unplugged, flow-through channels 320 and plugged channels 322, wherein the hydraulic diameter of the unplugged, flow-through channels 320 is larger than the hydraulic diameter of the plugged channels 322. In FIGS. 3C and 3D, the unplugged, flow-through channels are evenly distributed among the plugged channels, and vice versa.

The partial plugging patterns described above and variations thereof can be applied to one or both end faces of the honeycomb substrate (202 in FIG. 2A, 2B). For example, FIG. 4A shows a partial top view of the honeycomb substrate 202 if the partial plugging pattern of FIG. 3B is applied to both end faces of the honeycomb substrate 202. The unplugged, flow-through channels are designated 210 a. The plugged channels on one of the end faces of the honeycomb substrate 202 are designated 210 b. The plugged channels on the other of the end faces of the honeycomb substrate 202 (shown double hatched) are designated 210 c. In this example, the plugged channels 210 b, 210 c account for about 50% of the total channels in the honeycomb substrate 202 and the unplugged, flow-through channels 210 a are evenly distributed within the honeycomb substrate 202. FIG. 4A am 4B illustrates another partial plugging configuration wherein the outlet end is shown in FIG. 4B and the inlet end is shown in FIG. 4C. This embodiment illustrates a configuration with more plugged outlet channels than inlet channels. In particular, the channels plugged at the inlet end consist of about 25% of the total number of cell channels, whereas, at the outlet end, the plugged channels account for about 50% of the total number of channels. Channels plugged adjacent to the end shown are doubly hatched, whereas cells plugged adjacent the other end are singly hatched. Flow through channels are un-hatched.

FIGS. 5 and 6 illustrate additional embodiments of plug patterns that may be employed in the partial wall-flow filter. In these embodiments, the inlet end plugged cells are shown single hatched, the outlet end plugged cells are shown double hatched, and the flow-though cells are shown un-hatched. In these examples, about 50% of the cell channels are plugged and some of the plugged channels are located adjacent the inlet end and some are located adjacent the outlet end. Other combinations of plugged and unplugged cells in the partial wall-flow filter may be employed. For example, greater than 50%, or even greater than 60%, or even 75% or greater of the channels may be plugged, as compared to the total number of channels. The total porosity of the walls of the first filter may be greater than 45%, or even greater than 50%, greater than 55%, or even greater than or 60%. Preferred combinations excellent initial filtration efficiency and low pressure drop include greater than 50% plugged channels (as compared to the total number of channels), and total porosity of greater than 45%. Examples exhibiting relatively higher porosity, of greater than 60%, and which include greater than 50% (or even 60%) plugged channels exhibit relatively high ratios of filtration efficiency to pressure drop in both the clean and soot loaded states (see FIG. 10). Examples where the first filter is a partial flow filter having greater than 60% total porosity, greater than 50% plugged channels, some plugged channels adjacent to the inlet end and some adjacent to the outlet end and a relatively higher percentage of plugged channels at the outlet end achieve the best combination of initial filtration efficiency and low pressure drop (see the 25% Inlet, 50% Outlet 200/12/63% embodiment.

FIG. 7-10 are graphical plots illustrating experimental test examples that were investigated on the system and partial flow filter of the invention. As can be seen from the FIGS. 7-10, when the partial flow filter exhibits higher porosity and rear plugging, the combination of relatively high initial filtration efficiency (greater than 50%) can be achieved in the first filter while also exhibiting low pressure drop across the first filter. Filters having porosity of greater than 45%, or even 50%, or even 55%, or even 60% or higher are desired. In particular, the examples tested are outlined in Table 1 below. As can be seen, filters having 50% and 75% plugged channels (as compared to the total number of channels) and well as high (50%) and very high (63%) porosities. Several cell densities and wall thicknesses were also tested, as were inlet plugged, outlet plugged, and combinations of inlet and outlet plugged. In each case, the remainder of the channels not being deemed plugged were unplugged. In each case, the partial flow filter was matched with a 200/12 cell configuration, 50% porosity, 16 μm mean pore size filter with 50% of the channels conventionally plugged in a checkerboard pattern on each end. Certain examples of partial wall flow filters having porosity greater than 50%, greater than 10% of the plugged channels located adjacent the inlet end, and greater than 40% of the plugged channels located adjacent the outlet end exhibit excellent filtration efficiency and low pressure drop.

TABLE 1 Experimental Tests Cell Clean Ex. % Plugged Porosity Density T Wall % FE ΔP FE/PD No. Inlet/Outlet (%) (Cells/in²) (mil) Plugged Init % (kPa) (%/kPa) 1 50%/0%  50 200 12 50 15.8 0.74 21.4 2  0%/50% 50 200 12 50 41.7 0.92 45.3 3 25%/25% 50 200 12 50 27.8 0.72 38.6 4 50%/0%  63 200 12 50% 45 0.68 66.2 5  0%/50% 63 200 12 50% 61.1 0.87 70.2 6 50%/25% 50 200 12 75% 50 0.92 54.3 7 25%/50% 50 200 12 75% 48 1.09 44.0 8  0%/50% 63 300 8 50% 45.5 0.95 47.9 9 25%/50% 63 200 12 75% 73.7 0.91 81.0

FIG. 7 compares various test examples in a clean state, i.e., without soot loading (see points located inside the lower dotted outline) and in a soot loaded state (within the upper dotted outline) at 2 g/l soot loading of artificial soot. It was discovered the relatively higher porosity embodiments which combine greater than 50% plugged channels and more plugs adjacent the outlet end achieve excellent filtration efficiency coupled with low pressure drop. This allows relatively more soot to be captured within the first partial flow filter where such soot can be subjected to a higher relative extent of passive regeneration.

FIG. 8 illustrates additional examples of the partial wall-flow filter and shows the pressure drop for various embodiments as a function of grams per liter of soot loading. It should be recognized that certain embodiments exhibit little pressure drop increase as a function of soot loading. For example, the 50% rear plugged embodiment including greater than 60% porosity exhibits low pressure drop at even high soot loading rates (at 2 g/l and above).

FIGS. 9 and 10 illustrates that certain exemplary embodiments exhibit both high filtration efficiency is the first filter as well as low pressure drop. In particular, FIG. 10 plots the ratio of Filtration efficiency divided by pressure drop (in %/kPa) for various exemplary embodiments. In particular, several embodiment allow the capture of relatively lager amounts of soot in the first filter without a significant increase in pressure drop. For example, those embodiments exhibiting 60% or greater porosity and 50% or greater plugged channels and, more specifically, a larger amount of outlet plugged channels that inlet plugged channels, exhibit excellent filtration efficiency combined with low pressure drop. This a large portion of the soot may be subjected to passive regeneration in the first filter, thus requiring a lesser number of active regenerations intervals in the second filter.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. 

1. An exhaust system for venting exhaust from an engine through an exhaust line coupled to the engine, comprising: a first particulate filter disposed in the exhaust line and close-coupled with the engine, the first particulate filter operating in a first relative extent in a passive regeneration mode; and a second particulate filter positioned inline with, and spaced a distance from, the first particulate filter and operating to a second relative extent in a passive regeneration mode less substantially than the first relative extent.
 2. The diesel exhaust system of claim 1 wherein the first particulate filter exhibits an initial filtration efficiency of greater than 45%.
 3. The diesel exhaust system of claim 2 wherein the initial filtration efficiency is greater than 50%.
 4. The diesel exhaust system of claim 2 wherein the initial filtration efficiency is greater than 60%.
 5. The diesel exhaust system of claim 2 wherein the initial filtration efficiency is greater than 70%.
 6. The diesel exhaust system of claim 1 wherein the first particulate filter exhibits greater than 50% plugged channels as compared to a total number of channels.
 7. The diesel exhaust system of claim 6 further comprising greater than 60% plugged channels as compared to the total number of channels.
 8. The diesel exhaust system of claim 1 wherein the first particulate filter exhibits a total porosity of greater than 45%.
 9. The diesel exhaust system of claim 8 further comprising total porosity greater than 50%.
 10. The diesel exhaust system of claim 8 further comprising total porosity greater than 55%.
 11. The diesel exhaust system of claim 8 further comprising total porosity greater than 60%.
 12. The diesel exhaust system of claim 8 further comprising total porosity of 63% or greater.
 13. The diesel exhaust system of claim 1 wherein the first particulate filter exhibits: a total porosity of greater than 45%, greater than 50% plugged channels as compared to a total number of channels, and at least some unplugged channels.
 14. The diesel exhaust system of claim 13 wherein the total porosity is greater than 60%.
 15. The diesel exhaust system of claim 1 wherein the first particulate filter exhibits: a total porosity of greater than 60%, greater than 60% of the channels are plugged channels as compared to a total number of channels, and at least some unplugged flow-through channels.
 16. The diesel exhaust system of claim 1 wherein the first particulate filter exhibits: a total porosity of greater than 60%, greater than 50% of the channels are plugged channels as compared to a total number of channels, at least some unplugged flow-through channels, and some of the plugged channels are plugged adjacent to an inlet end and some of the plugged channels are plugged adjacent to an outlet end.
 17. The diesel exhaust system of claim 1 wherein the first particulate filter includes some unplugged flow-through channels and some plugged channels wherein at least some of the plugged channels are plugged adjacent to both an inlet end and an outlet end.
 18. The diesel exhaust system of claim 17 wherein a percentage of plugged channels adjacent to the inlet end is less than a percent of plugged channels adjacent to the outlet end.
 19. The diesel exhaust system of claim 1, wherein the first particulate filter comprises a ceramic honeycomb substrate comprising a plurality of channels, wherein a first percentage of the channels are plugged and a second percentage of the channels are open and wherein the number of plugged channels divided by a total number of channels, and expressed as a percentage, is greater than 50%.
 20. The diesel exhaust system of claim 19 wherein the percentage of plugged channels to total channels is greater than 60%.
 21. The diesel exhaust system of claim 1 wherein the first filter includes an inlet end and outlet end, and the plugged channels are positioned only adjacent the outlet end.
 22. The diesel exhaust system of claim 1 wherein the honeycomb substrate has an inlet end and an outlet end, and the plugged channels include plugs adjacent to both the inlet end and the outlet end.
 23. The diesel exhaust system of claim 1 wherein the first filter plugged channels and unplugged flow-through channels and a hydraulic diameter of the plugged channels is different than a hydraulic diameter of the unplugged flow-through channels.
 24. The diesel exhaust system of claim 1 wherein the first particulate filter is positioned downstream of a turbocharger disposed in the exhaust line.
 25. The diesel exhaust system of claim 1, further comprising a diesel oxidation catalyst positioned between the first particulate filter and the second particulate filter.
 26. The diesel exhaust system of claim 1 wherein the second particulate filter is positioned inline with, and spaced a distance (d) of greater than or equal to 12 inches (30.5 cm) from the first particulate filter.
 27. The diesel exhaust system of claim 1 wherein the first particulate filter includes a first inlet temperature (T₁) and the second particulate filter includes a second inlet temperature (T₂), wherein a ratio of inlet temperatures (T₁/T₂) is greater than or equal to 1.1.
 28. The diesel exhaust system of claim 1 wherein the first particulate filter is positioned at a distance from the engine wherein during operation, the inlet temperature (T₁) is at a temperature above 250° C. for a some portion of the operation.
 29. A diesel exhaust system, comprising: a first particulate filter close-coupled with the exhaust manifold, the first particulate filter having a first percentage of plugged channels and a second percentage of unplugged flow-through channels; and a second particulate filter positioned inline with and spaced a distance (d) of greater than or equal to 12 inches (30.5 cm) from the first particulate filter and including only plugged channels such that the first filter exhibits passive regeneration to a substantially greater extent than the second filter.
 30. The diesel exhaust system of claim 29 wherein the second particulate filter is a wall flow filter including only plugged channels.
 31. The diesel exhaust system of claim 29 wherein the first particulate filter includes plugged channels and unplugged flow-through channels, and the plugged channels include some channels that are plugged adjacent to an inlet end and other channels that are plugged adjacent to an outlet end.
 32. A method of operating a diesel exhaust system, comprising the steps of: passing an exhaust gas through a first particulate filter disposed in an exhaust line wherein the first particulate filter includes a first inlet temperature (T₁), the first particulate filter having a first percentage of plugged channels and a second percentage of unplugged flow-through channels wherein the exhaust gas is first filtered; and passing the first filtered exhaust gas through a second particulate filter positioned inline with and spaced a distance from the first particulate filter wherein the second particulate filter includes a second inlet temperature (T₂), wherein the first exhaust gas undergoes a second filtering and a ratio of inlet temperatures (T₁/T₂) is greater than or equal to 1.1.
 33. A partial wall-flow filter, comprising: plugged channels and unplugged, flow-through channels wherein the plugged channels include some channels that are plugged adjacent to an inlet end and other channels that are plugged adjacent to an outlet end.
 34. The partial wall-flow filter of claim 33, further comprising a percentage of plugged channels to total channels greater than 50%.
 35. The partial wall-flow filter of claim 33, further comprising a total porosity of greater than 45%.
 36. The partial wall-flow filter of claim 33, further comprising a total porosity of greater than 50%.
 37. The partial wall-flow filter of claim 33, further comprising a total porosity of greater than 60%.
 38. The partial wall-flow filter of claim 33, further comprising a total porosity of greater than 50%, and greater than 10% of the plugged channels located adjacent to an inlet end and greater than 40% located adjacent to the outlet end.
 39. The partial wall-flow filter of claim 33, further comprising wherein a number of channels plugged adjacent to the inlet end is less than a number of channels that are plugged adjacent to the outlet end.
 40. The partial wall-flow filter of claim 33, further comprising wherein a number of channels plugged adjacent to the outlet end is less than a number of channels that are plugged adjacent to the inlet end.
 41. The partial wall-flow filter of claim 33, further comprising an oxidation catalyst residing on a porous wall of the partial wall-flow filter. 